There are many industrial and clinical applications requiring a high flux of thermal neutrons. A neutron is considered to be thermal when it is in thermal equilibrium with the surrounding materials. Thermal neutrons have a Maxwellian distribution of energies and can be generally considered to have a kinetic energy less than 1 eV (electron-volt). Examples of industrial applications include neutron radiography and Prompt Gamma Neutron Activation Analysis (PGNAA). Some examples of clinical applications include production of radioactive stents used in the prevention of restenosis following arterial intervention, such as balloon angioplasty, and production of short lived radioisotopes used in radiation synovectomy or brachytherapy.
Hampering the continued development of these applications is often the lack of a suitable neutron source. The highest thermal neutron fluxes are produced in nuclear research reactors. These facilities, however, are few in number and often lack the clinical environment necessary for medical research. Other types of neutron sources include radioisotope sources, fusion sources, cyclotrons, and ion accelerators. Much work has gone into the development of these neutron sources with many variations in each category. However, a neutron source that has a high thermal flux suitable for installation in industrial or clinical environments is not generally available. Furthermore, the cost of many of these systems is beyond the reach of many institutions that could make use of the technology.
Another known method of producing neutrons is with an electron accelerator fitted with an x-ray converter and a photoneutron target. In one system, a high power (1 MW) continuous current electron accelerator is used to generate a 30 MeV electron beam, which is incident on a Tungsten target of the x-ray converter. The resulting bremsstrahlung photons are then directed to a tank of heavy water, thereby producing high energy neutrons (up to 14 MeV). While this system may maximize the photoneutron yield, the energy of these neutrons is too high to be thermalized effectively. Such high energy photons and neutrons also requires a massive thickness of biological shielding. Moreover, the high power electron accelerator would make the system relatively large, extremely expensive to build and to operate, and would stretch the technical expertise of a typical radiology department. These types of electron accelerators are primarily used for research and do not have the reliability required for use in a clinical setting.